This invention relates generally to a system for measuring axial displacement in a rotating environment, and more specifically, to the measurement of relative axial movement between adjacent components in a gas turbine rotor assembly.
In an advanced gas turbine power plant developed by the assignee of this invention, the stage 1 and 2 gas turbine buckets are closed circuit cooled with steam or air fed by a delivery system extending through the gas turbine rotor assembly. It is important to evaluate wear resulting from relative axial movement between the axial delivery tubes and the rotating turbine wheels. The rotor for which this measurement is required is approximately 59 inches in diameter and has a maximum rotational speed of 4200 rpm. The steam delivery tubes are buried deep within the massive rotor assembly and the sensors used to measure the axial displacement of these tubes must be mounted in an area which is approximately 24 inches from the machine center line, i.e., the rotor axis, and therefore must withstand repeated exposure to a centrifugal acceleration of over 12,000 g""s. In addition, the temperature of the component to be measured will range from 75xc2x0 F. to 250xc2x0 F. and the area through which the signal leads must pass will be nearly 1000xc2x0 F. The system must survive a series of 40-minute cycles to maximum speed and temperature; a life expectancy which exceeds 5000 cycles is desirable. The unit may be either self-powered or run from an external power source through a rotating power coupling device. The displacement signal will be transferred to a stationary platform through a rotating telemetry system capsule.
In accordance with an exemplary embodiment of the invention, a commercially available Bentley Nevada proximity transducer system is employed to sense the axial displacement measurement of four axially oriented steam supply cooling tubes arranged at 90xc2x0 intervals about the rotor axis relative to stationary components in the rotor assembly, while the tubes and stationary components are rotating about the rotor axis.
The measurements are taken with transducers having no moving parts to jam, wear out or degrade, and the standard transducer probe is rated to a maximum temperature of 350xc2x0 F. which is slightly above the expected temperature in the immediate area of the measurement. The transducers or sensors are oriented normal to the direction of motion of each steam delivery tube, concentrating on a measurement site which, in this case, is a sloped or curved region (or fillet radius) on the tube. This allows the tube to stay within the measurement distance range of the probe. Custom cabling is connected to the circumferentially spaced sensors, the cabling including mineral insulated metallic sheathed sections which will span the hot area, and lower temperature flexible sections for connection to the probes at one end and to oscillator/demodulator modules at the other end.
All of the oscillator/demodulator modules rotate near the machine center line in a module support ring supported by two permanently lubricated ball bearings between the machine bearing housing and the telemetry capsule. The modules are powered via a rotating induction power system mounted to the OD of the module support ring. Rectifiers, filters, and regulators within the induction power system rotating ring provide the 24 VDC required by the modules. The 2 to 18VDC output signals from the modules connect to the telemetry capsule via rotating circular connectors and a flexible coupling arrangement. The telemetry capsule samples the signals at a rate of approximately once per second and then digitally transmits the data to the ground station across a radio frequency link. The test cell data acquisition system then receives the digital data from the ground station via a serial link.
Due to the shape of the fillets on the tubes used as the targets for the measurement system, the output of the system will be inherently non-linear. For this reason, a detailed calibration must be performed in situ prior to running if an accurate absolute measurement of the tube movement is desired. Following the calibration, a curve-fit may be made and the resultant coefficients inserted into the data acquisition computer for engineering unit calculation.
Accordingly, in its broader aspects, the present invention relates to a measuring system for measuring axial displacement of a tube relative to an axially stationary component in a rotating rotor assembly having a longitudinal axis, the measuring system comprising at least one displacement sensor adapted for placement adjacent a longitudinal axis of the tube; an insulated cable system adapted for passage through the rotor assembly; an oscillator/demodulator module assembly located axially beyond the rotor assembly to which the cables are connected; and a telemetry system operatively connected to the module assembly for sampling signals from the module assembly and forwarding data to a ground station.